Was there an "insert" in that letter explaining that the due date is July 15 and the computers couldn't be stopped from sending out those notices (or something to that effect)? See https://proconnect.intuit.com/community/tax-talk/discussion/irs-bills-look-at-the-insert-not-the-not...

Checks sent in April apparently haven't been cashed. I just heard from another client. He sent by certified mail. Everything is piled up in mail trailers.